Amsterdam toast update

[hbharding]

Summary

Updates Amsterdam theme for EuiToasts:

• Titles are now bold
• Removed 1px light grey border surrounding toast
• Increased vibrancy of top-border color
• Reduced spacing between title and content from 12px to 8px, which is consistent with EuiText's 8px spacing after headers. This change applies to both our existing base theme and Amsterdam.

Screenshot After:

Screenshot Before:

Checklist

• Check against all themes for compatibility in both light and dark modes
• [ ] Checked in mobile
• [ ] Checked in Chrome, Safari, Edge, and Firefox
• [ ] Props have proper autodocs
• [ ] Added documentation
• [ ] Checked Code Sandbox works for the any docs examples
• [ ] Added or updated jest tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4076/

[cchaos]

Initially I thought some more of these changes could be added to the default theme as well, but I think ultimately we should continue to set apart Amsterdam and not just blanket updates. So 👍 LGTM, with just a comment about the CL.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4076/